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Rh which had gradually come to the front as the most important scientific problem there was at the time, and which no one could solve. It was this. If gravity really changed with the distance as Newton had suspected fourteen years before, and as many others had since suspected from study of Kepler's great Third Law, then would Kepler's Second Law follow as a consequence, that is to say would the planets all move in ellipses round the Sun? People felt that probably this was a necessary consequence, but no one could prove it. Hooke claimed to have proved it, but no one now believes his claim. And he accused Newton of not knowing the fact, much less being able to prove it. This made Newton very angry. He first sat down and proved the proposition for his own satisfaction—did what every one was trying to do and could not—and then he tossed the matter aside, as he could not bring himself to reply to the disagreeable man who had wrongly accused him. And so things might have remained had it not been for Edmund Halley, a man of a very different kind, who came in the nick of time to rescue this great achievement of Newton's. Halley was an Oxford man, Newton was at Cambridge: Oxford and Cambridge often meet in friendly rivalry, but this time they met in co-operation, and took each a share in the great discovery of Gravity, though the share of Cambridge is of course much the greater. Halley had tried without success to get an answer to the difficult question whether planets moved in ellipses because the Sun attracted them in the way suspected; and as a forlorn hope he travelled all the way to Cambridge to ask Newton about it. To his